The present invention relates to an axial flux type coreless-brushless motor in which a stator yoke, a driving coil block, a rotor magnet and a rotor yoke are sequentially stacked up.
In this kind of coreless-brushless motor, the driving coil block is formed by laminating a plurality of sheet coils with an insulating layer interposed between adjacent ones of them as disclosed, for example, in U.S. Pat. No. 4,340,833 and EPC Publication No. 30,008 (June 10, 1981). The sheet coils each have a plurality of spiral coils formed as conductor patterns at equiangular intervals on at least one side of an insulating sheet. For interconnecting the sheet coils of the driving coil block, it is the general practice in the prior art to lead out two leads from each sheet coil and to solder such leads of the respective sheet coils for interconnection. The leads are soldered outside the laminated sheet coils, but the space that is occupied by the soldered connecting blocks in the motor increases with an increase in the number of sheet coils used. In other words, the space in the motor is unnecessarily consumed by the connecting portions which do not directly serve to produce the motor driving force.
In the coreless-brushless motor, the rotational angular position of a rotor magnet is detected and, by the detected output, each phase current of a multi-phase driving coil is controlled to be turned ON and OFF. A Hall element has been employed for detecting the rotational angular position of the rotor magnet. An ordinary Hall element is produced by connecting leads to a Hall element pellet and then molding it with resin, and even a miniaturized element is about 2 mm thick. Since such a Hall element is disposed between the stator yoke and the rotor magnet, there is a limit to the reduction of the magnetic air gap between the rotor magnet and the stator yoke, imposing limitations on the reduction of the thickness of the motor and resulting in lowered efficiency.
Driving devices for a floppy disc, a VTR and so forth must be controlled in speed with a relatively high degree of accuracy. One method that has been employed to meet this requirement is to provide a frequency generating coil (hereinafter referred to as the FG coil) between the stator yoke and the rotor magnet to obtain an AC signal of a relatively high frequency proportional to the revolving speed of the rotor magnet. With this method, even if the rotating speed of the rotor magnet is low, an AC signal of relatively high frequency is obtained, by which the speed control can be effected with high accuracy, and the motor can be made smaller than in a case where the FG coil is disposed outside the motor case. However, since the FG coil responds to magnetic fluxes from the rotor magnet to generate an AC signal and also responds to magnetic fluxes from the driving coil, the output of the FG coil contains components of a low frequency AC signal caused by the magnetic flux of the driving coil and a high frequency AC signal caused by the magnetic flux of the rotor magnet superimposed on each other. As a result of this, in a case of detecting the frequency, that is, the revolving speed of the rotor magnet, as the frequency of zero level crossing pulses of the output AC signal from the FG coil, some of the AC signals based on the magnetic flux of the rotor magnet do not cross the zero level owing to the abovesaid superimposition, making correct speed detection impossible.